1. Field of the Invention
The present invention relates to a zoom lens used in TV cameras and the like.
2. Related Background Art
There are zoom lenses having the function of limiting a driving range of a zoom portion (which is so called the tracking function). With the zoom lenses developing their wider-angle tendency and higher-magnification tendency through the years, there are circumstances where a person or the like unwanted to take around a subject is present in the field with the zoom portion on the wide-angle side, whereas there are also circumstances where the subject is magnified so as to be too large to be settled in the field with the zoom portion at the telephoto extreme. Under such circumstances, a lens operator always has to finely adjust zoom stop positions on the wide-angle side and on the telephoto side by a zoom demand.
Thus the tracking function becomes useful for limiting the driving range of the zoom portion and permitting such photography of picture that only the subject desired to take is photographed at the wide-angle extreme and that the entire image of the subject is just fitted in the field at the telephoto extreme, without the need for the fine adjustment by the zoom demand.
There are also zoom lenses having the zoom preset function. In the case of news report programs, baseball relay broadcasting, etc., there are such occasions that a specific person is repeatedly taken many times in a specific size. The operator needed to set the zoom at the position for the photography in the specific size by manipulating the zoom demand every such occasion.
When the zoom preset function is used on such occasions, any given zoom position can be stored, and the zoom portion can be set at the stored zoom position by manipulating a switch as necessary, whereby the specific person can be photographed in the specific size.
FIG. 5 shows a control flowchart in a method of controlling the positions of lenses constituting the zoom portion and the focus portion in a zoom lens having the tracking function and the zoom preset function.
In this zoom lens, each of zoom preset data and tracking limit data from the zoom demand is converted into counter-equivalent position data (steps 103 to 106); the position data after the conversion is compared with values of the counters and arithmetic operations are carried out (steps 107 to 110); and a motor is driven to control the position of each lens (step 111).
In the control using the position data of the lenses as described above, however, when the driving range of the zoom portion is limited by the tracking function, the angle of view at the tracking end varies depending upon the lens position of the focus portion, because the angle of view varies with driving of the focus portion.
In use of the zoom preset function, similarly, if the focus position upon storage of the lens position of the zoom portion is different from that upon setting of the zoom position by use of the preset function, the angle of view after the preset operation will be different from that upon the storage.
An object of the present invention is, therefore, to provide a zoom lens capable of keeping the angle of view constant under use of the tracking function and/or the zoom preset function, regardless of the focus driving.